1. Technical Field of the Invention
The present invention relates to a reflection liquid crystal display such as a display for a mobile terminal device, a terminal display for utilizing various types of media for individuals, a display of a mobile telephone, and a display in an amusement device such as a game machine, a method for producing the same, and a method for driving the same. More specifically, this invention relates to a reflection liquid crystal display having excellent field-of-view angle characteristics, the production process of which is facilitated, a method for producing the same and a method for driving the same.
2. Description of the Related Art
Recently, demand for a reflection liquid crystal display, power consumption of which is reduced, has increased in line with development and diversification of mobile devices. Such a display that enables color display has been increasingly desired for use in, particularly, mobile telephone sets, mobile terminals, and office automation devices. A brighter display is further required in view of mobility, and reasonably wide field-of-view angle characteristics are required. Especially preferable, since narrow field-of-view angle characteristics are desired in an individual use although wide field-of-view angle characteristics are desired where a plurality of people observes a display, a reflection liquid crystal display that enables a changeover between a wide field-of-view angle and a narrow field-of-view angle is desired.
Conventionally, such types are widely used in reflection liquid crystal displays, in which a polarization plate is used as in a supertwisted nematic type (STN type) or twisted nematic type (TN type) that has been widely used in transmission type liquid crystal displays. In the case of reflection type, only a single polarization plate is used, differing from the transmission type. However, since it is necessary to switch reflection light, such a type is common, which is a so called single polarization plate used along with a quarter-wavelength plate as described in, for example, T. Sonehara et al., Japan Display 1989, Page 192 (1989).
A description is given of a display principle of a single polarization plate type, taking a normally white type, which has been most widely used, as an example (Prior art example 1). FIG. 1A and FIG. 1B are views showing the display principle of a reflection type liquid crystal element of a prior art single polarization plate type, wherein FIG. 1A shows an example in the case of white display, and FIG. 1B shows an example in the case of black display. Also, FIG. 1A and FIG. 1B show only optical elements of the reflection liquid crystal display.
As shown in FIG. 1A, incident non-polarized light 1107 is brought into collision with a reflection plate 1104 after having passed through a quarter-wavelength plate 1102 of a wide-band and passed through a liquid crystal layer 1103 and returns in its inverted optical route, and, when the light passes through the polarization plate 1101, it enters eyes of people, wherein the light can be recognized as an image. At this time, by changing the state of polarization by utilizing electro-optical effects of liquid crystal, switching of the reflection light can be carried out. First, the incident non-polarized light 1107 enters the polarization plate 1101 and is converted to light polarization having a specified vibration direction. At this time, the polarization plate is set so that the outgoing light becomes P-polarized light 1105. And, if the optical axis of the quarter-wavelength plate 1102 is disposed so that it forms 45 degrees with respect to the transmission axis of the polarization plate, the light that has passed through the quarter-wavelength plate 1102 becomes rightward circular-polarized light 1106 and enters the liquid crystal layer 1103. In either the TN mode or the STN mode, retardation of the liquid crystal 1103 is set so that it gives xcex/4, that is, a phase difference xcfx80/2 when no voltage is applied. Therefore, light that has passed through the liquid crystal layer 1103 again becomes P-polarized light 1105 and reaches the reflection plate 1104. In the reflection, since the P-polarized light 1105 is reflected as it is P-polarized light 1105, it returns in a completely inverted optical path of the path along which it entered, and is converted to rightward circular-polarized light 1106 by the liquid crystal layer 1103. Further, the light becomes P-polarized light 1105 by the quarter-wavelength plate 1102 and is caused to radiate from the polarization plate 1101 as it is P-polarized light 1105. That is, white display is enabled in a state where no voltage is applied onto the liquid crystal.
As shown in FIG. 1B, if voltage is applied to the liquid crystal layer 1103 and liquid crystal is erected so that the liquid crystal molecules become perpendicular with respect to its substrate, the retardation of the liquid crystal layer 1103 becomes almost zero, and a phase difference 0 is given. That is, the liquid crystal layer 1103 does not give any influence to the state of polarization. In this state, where incident non-polarized light 1107 enters the polarization plate 1101, light that has passed through the polarization plate 1101 and quarter-wavelength plate 1102 becomes rightward circular-polarized light 1106 as described above. Herein, since voltage is applied onto the liquid crystal layer 1103, the liquid crystal layer 1103 does not change the state of polarization, and the rightward circular-polarized light 1106 passes through the liquid crystal layer 1103 as it is the rightward circular-polarized light 1106 and enters the reflection plate 1104. Since the advancement direction of the light is inverted by reflection, the rightward circular-polarized light 1106 becomes a leftward circular-polarized light 1108 and returns. Since the liquid crystal layer 1103 also does not change the state of polarization, light that passed through the liquid crystal layer 1103 enters the quarter-wavelength plate 1102 as it is a leftward circular-polarized light, and it becomes S polarization 1109 whose direction of polarization is different by 90 degrees from the P-polarized light 1105, wherein the light enters the polarization plate 1101. Since the transmission axis of the polarization plate 1101 is set so that it can make the P-polarized light pass therethrough, wherein the S polarization 1109 cannot pass through the polarization plate 1101, and it is displayed as black. Depending upon the intensity of application voltage, the retardation of the liquid crystal layer 1103 can be varied, wherein intermediate colors can be displayed.
Also, Japanese Unexamined Patent Application No. Hei-10-20323 (hereinafter called a xe2x80x9cprior art example 2xe2x80x9d) has disclosed a liquid crystal display whose production is facilitated and field-of-view angle characteristics are excellent. In the prior art example 2, a liquid crystal layer in which two or more types of slight areas coexist is placed between two substrates, and has an electrode having an opening formed on at least one substrate, and a second electrode (control electrode) secured in the opening, wherein a voltage that is higher than the voltage applied between the electrode having the opening and the electrode opposed thereto is applied between the control electrode and the electrode opposed thereto, thereby securing a wide field-of-view angle.
Further, Japanese Unexamined Patent Publication No. Hei-7-239471 (hereinafter called a xe2x80x9cprior art example 3xe2x80x9d) discloses the use of a cholesteric material layer and phase plate, that act as a reflection layer, for the purpose of improving the brightness and color purity of a reflection liquid crystal display. In the prior art example 3, the upper and lower substrates are disposed so as to be opposed to each other, a liquid crystal layer is placed and secured between these two substrates, and the prior art example 3 comprises a phase plate disposed on the opposite side of the upper substrate liquid crystal layer, an upper polarization plate disposed further thereon, a cholesteric material layer that is disposed on the surface of the lower substrate opposed to the upper substrate and is disposed between the surface and the liquid crystal layer, and an optical absorption layer formed at the opposite side of the lower substrate liquid crystal layer. Thus, the cholesteric material layer is formed in liquid crystal cells and is used as a color filter, whereby shadows in the dark display portion can be removed.
In addition, a liquid crystal display in which a wide field-of-view angle and a narrow field-of-view angle are changed over is disclosed in Japanese Unexamined Patent Application Nos. Hei-6-59287 and Hei-10-197844 (hereinafter respectively called a xe2x80x9cprior art example 4xe2x80x9d and a xe2x80x9cprior art example 5xe2x80x9d).
In the prior art example 4, the field-of-view angle of transmission liquid crystal cells is changed over by adjusting the outgoing light, utilizing a guest-host liquid crystal or grating. Also, in the prior art example 5, such a method is disclosed, in which the reflection type and transmission type are changed over by utilizing transmission and dispersion of macromolecular dispersion liquid crystal, and the changeover of the field-of-view angle of a liquid crystal display is carried out by utilizing the guest-host liquid crystal.
However, in the case of displaying by means of the mode of prior art example 1, as has been made clear in FIG. 1A, since the light that enters the liquid crystal layer 1103 in bright display is based on linear polarization, in the TN mode or STN mode, it is necessary to set the rubbing direction and polarization direction, so that they are made coincident with each other or different by 90 degrees from each other, in order to obtain a high transmittivity, and it is necessary to accurately control the rubbing direction and arrangement of the polarization plate 1101 and a wide-band quarter-wavelength plate 1102. In addition, if a perpendicular orientation mode and amorphous TN mode, which can shorten the production process without requiring any rubbing, are used, a dark display portion is securely produced in a bright display state, wherein sufficient brightness cannot be obtained. Further, another problem occurs in that, since the field-of-view angle is unitarily determined by design of a reflection plate, the wide field-of-view angle and narrow field-of-view angle cannot be changed over.
Still further, in the art described in the prior art example 2, such problem occurs in that a voltage must be applied to the second electrode in order to control its drive and a wide field-of-view angle and narrow field-of-view angle cannot be switched.
In addition, in the art described in the prior art example 3, in the phase plate, no consideration is taken with respect to the relationship between the direction of liquid crystal orientation and the direction of polarization of reflection light that enters the liquid crystal layer in the TN and STN modes, and no device is provided so that brightness can be secured in regard to fluctuations of a process such as a rubbing direction, etc. In particular, even if a perpendicular orientation mode or amorphous TN mode, which can shorten the production process not requiring any rubbing, is used, no consideration is provided with respect to a method and/or construction of securing sufficient brightness. Therefore, sufficient brightness cannot be secured by such a simple process. Also, where the cholesteric material layer is used as a reflection layer, since the range of field-of-view angle that is capable of observing selection reflection is narrow, it is necessary to further widen the field-of-view angle in practice. However, a problem occurs in that a wide field-of-view angle and a narrow field-of-view angle cannot be changed over.
Still further, in the prior art example 4, it is disclosed only that the outgoing light is adjusted by utilizing a guest-host liquid crystal or grating in order to adjust the field-of-view angle of transmission liquid crystal cells. No disclosure is provided with respect to a reflection type. Furthermore, where the field-of-view angle is changed over by the system disclosed in the prior art example 4, a problem occurs in that, although it is possible to only narrow and/or limit, in a certain area, the field-of-view angle of liquid crystal cells to be used, it is not possible to widen the field-of-view angle of the liquid crystal cells.
In addition, in the prior art example 5, as in the prior art example 4, although it is possible to only narrow and/or limit, in a certain area, the field-of-view angle of liquid crystal cells to be used, it is not possible to widen the field-of-view angle of the liquid crystal cells. Therefore, it is necessary to use a mode in which the field-of-view angle of the liquid crystal cells is wide. A liquid crystal mode having practical brightness and high contrast in the reflection type is only a single polarization plate type of TN as in the prior art example 1. However, another problem occurs in that the field-of-view angle in the mode is narrow, and a practically sufficient field-of-view angle cannot be obtained by the system by which the field-of-view angle is further narrowed.
Therefore, as described above, it is difficult to secure sufficient brightness by a single polarization plate type in which a conventional TN or STN mode is employed. Still further, it is necessary to accurately control the rubbing direction, etc., and tolerance is narrow with respect to fluctuations in the process. In particular, it is impossible to secure sufficient brightness with respect to the perpendicular orientation not requiring any rubbing and the mode of amorphous TN, etc. In addition, still another problem occurs in that the necessary field-of-view angle in practice cannot be secured, and the field-of-view angle cannot be changed over.
It is therefore an object of the invention to provide a reflection liquid crystal display, having an excellent field-of-view angle, which can display brightly with excellent color purity in a mode of perpendicularly-oriented liquid crystal and amorphous TN, etc., which does not require any rubbing, and can be easily produced, and in which a wide field-of-view angle and a narrow field-of-view angle can be simply changed over, a method for producing the same, and a method for driving the same.
A reflection light crystal display according to the invention comprises: a first substrate; a second transparent substrate disposed at the forward side in the incident direction of light so that it is opposed to the first substrate; a liquid crystal layer secured and placed between said first substrate and second substrate; a color filtering layer consisting of a cholesteric material layer provided between said first substrate and liquid crystal layer; a light absorbing layer secured rearward of said color filtering layer in the incident direction of light at said first substrate side; a quarter-wavelength plate secured at the second substrate side; and a polarization plate disposed further forward of the incident direction of light than the quarter-wavelength plate.
Another reflection liquid crystal display according to the invention comprises: a first substrate; a second transparent substrate disposed forward of the incident direction of light so that it is opposed to the first substrate; a liquid crystal layer secured and placed between said first substrate and second substrate; a color filtering layer consisting of a cholesteric material layer provided between said first substrate and said liquid crystal layer; a light absorbing layer secured rearward of said color filtering layer in the incident direction of light at said first substrate side; a three-color cholesteric material layer, which is provided at the second substrate side and has an inverted twist of that of said cholesteric material layer.
In the present invention, light that enters the liquid crystal layer is subjected to circular polarization without fail, the intensity of light radiating from the liquid crystal layer is not influenced by the direction of orientation of liquid crystal on the plane parallel to the substrate surface. Therefore, a bright white display is enabled regardless of the direction and angle of the liquid crystal. As a result, in the TN or STN mode, even if the rubbing direction slips in the process of production, this does not influence the display at all. In addition, in modes not requiring any rubbing such as the perpendicular orientation mode or amorphous TN mode, blackening portions will not be produced in the white display as in the prior arts. Therefore, a remarkably bright display is enabled in comparison with the prior arts. Further, it is enough that the liquid crystal layer has a function by which retardation (dxcex94n), which is a product obtained by multiplying the refractive index anisotropy xcex94n of liquid crystal molecules by the thickness d of the liquid crystal layer, is changed by xcex/2 (with a phase difference xcfx80). Therefore, even in a high-speed mode other than the abovementioned, this can be used without any necessity of accurate control in the rubbing direction.
Also, in the present invention, it is preferable that the invention is provided with a scattering layer that scatters light forward of the incident direction of light of said polarization plate or forward of the incident direction of light of said three-color cholesteric material layer. Said scattering layer has two transparent electrodes disposed so as to be opposed to each other and a macromolecular dispersion liquid crystal layer placed and secured between these transparent electrodes, and may be such a type that can change over transmission and dispersion of said macromolecular dispersion type liquid crystal layer by applying voltage to said macromolecular dispersion liquid crystal layer. A wide field-of-view angle and a narrow field-of-view angle can be changed over by attaching or detaching a scattering film or switching the macromolecular dispersion liquid crystal layer. In particular, in the present invention, since a color filtering layer consisting of a cholesteric material layer is provided at the side where the liquid crystal layer of the first substrate exists, a problem of so-called parallax does not occur. In addition thereto, in the invention, since the color filtering layer consisting of a cholesteric material layer is provided at the side where the liquid crystal layer of the first substrate exists, and the direction of selection reflection of the cholesteric material is limited to a specified direction, only automatically collimated light is reflected on the scattering layer. Therefore, no problem in view of parallax does occur, wherein it is possible to obtain a wide field-of-view angle. To secure a narrow field-of-view angle, such adjustment may be carried out, by which the scattering is removed or the angle of dispersion is reduced. In the case where only a reflection plate consisting of a cholesteric material layer is used, a narrow field-of-view angle limited in only the direction of selection reflection can be made into a wide field-of-view angle which is sufficient in practice, by using a scattering film or a macromolecular dispersion liquid crystal layer.
Further, the invention may be provided with a plurality of scanning signal lines secured on the surface of said first substrate opposed to said second substrate and a plurality of picture signal lines disposed on these scanning signal lines in the form of a matrix, a plurality of thin-film transistors formed so as to correspond to the intersection of said scanning signal lines and said picture signal lines, at least one pixel that is constituted by an area surrounded by said plurality of scanning signal lines and picture signal lines, pixel electrodes that are connected to said thin-film transistor corresponding to respective pixels and are formed rearward of said liquid crystal layer in the incident direction of light, and a common electrode that is formed forward of said liquid crystal layer in the incident direction of light and applies a reference voltage to said plurality of pixels. Thereby, since pixel electrodes are disposed between the color filtering layer and liquid crystal layer, alignment between the color filtering layer and pixel electrodes is no longer required, wherein the overlapping accuracy of the first and second substrates can be remarkably relieved. Still further, since the pixel electrodes are disposed between the color filtering layer and the liquid crystal layer, influence of an electric field in the lateral direction from the scanning signal electrodes and picture signal electrodes can be remarkably relieved.
In addition, in at least either one of said scanning signal electrode or said picture signal electrode, a part of said pixel electrodes or a shielding electrode may be disposed forward in the incident direction of light. Thus, in the case of an active matrix liquid crystal display, by disposing a shielding electrode above at least either one of the scanning signal electrode or picture signal electrode, no influence is received in the lateral direction from the scanning signal electrode and picture signal electrode.
Also, the above pixel electrodes may be circular or equilaterally polygonal to have more sides than those of a triangle, and said common electrode has a larger area, when being observed from right above, than that of said common electrode, and is formed at a position where it can cover up the entirety of said pixel electrode. Further, said pixel electrode is shaped so that a plurality of circles or equilateral polygons which have more sides than those of a triangle range one after another while the common electrode may have a larger area, when being observed from right above, than that of said pixel electrode and is formed at a position where it covers up the entirety of said pixel electrode. Still further, said common electrode may be formed on almost the entire surface of said second substrate.
In addition, said pixel electrode is provided with cuts formed at equidistant positions on the circumference or at the corners of a polygon, or may be provided with projections protruding outward at equidistant positions on the circumference or at the corners of a polygon, or a recess may be formed at a part of said pixel electrode.
Thereby, it is possible to orient and divide liquid crystal cells as necessary, in view of the field-of-view angle characteristics when being used in a narrow field-of-view angle, uniformity of the brightness on the panel surface, and response rate, etc. As described above, where voltage is applied between such electrodes, oblique electric fields are produced up and down with symmetry well secured. For example, in liquid crystal having a dielectric constant of negative anisotropy (dielectric anisotropy, which is perpendicularly oriented, the number of directions along no voltage is applied, said liquid crystal layer in the respective pixels may have two or more types of minute areas in which the rise directions of liquid crystal molecules differ from each other, or said liquid crystal layer in the respective pixels may have two or more types of minute areas in which the twisting directions of the liquid crystal molecules differs from each other, or said liquid crystal layer in the respective pixels may have four types of minute areas in which the twisting direction and rise direction of liquid crystal molecules differ from each other. In this case, it is preferable that the pre-tilt angle of the liquid crystal molecules in the boundary phase between said first substrate and second substrate is 1 degree or less.
In this case, the pixel electrode is shaped so as to have good symmetry, the common electrode covers the entirety of the upper part of the pixel electrode when being observed from above, and is formed so that its area becomes larger than that of the pixel electrode, wherein, when voltage is applied between the pixel electrode and the common electrode, an oblique electric field is produced in the liquid crystal layer with good symmetry secured due to the shape characteristics of the upper and lower electrodes. Although there is a possibility that both rightward twist and leftward twist are generated in respective parts of the pixels, one of the twisting directions is preferentially produced in an area where respective pixels are divided, and the state of orientation is automatically produced, and in the case of twisted nematic orientation, pixel division which liquid crystal molecules fall down becomes two or more, wherein it becomes possible to smoothly orient and divide liquid crystal in pixels. That is, a boundary of division is produced at the center of a pixel due to an oblique electric field that is naturally produced, and liquid crystal molecules are caused to fall down from the end of the pixel electrode toward the middle. If the shape of the pixel electrode is made symmetrical, liquid crystal molecules are caused to naturally fall down from respective sides of the pixel electrode toward the middle thereof. Therefore, they are naturally divided. Polygons are not necessarily made equilateral, wherein some deformation may be permitted.
Further, said liquid crystal layer may include macromolecular organic compounds.
Still further, in said liquid crystal layer, the anisotropy of the dielectric constant of liquid crystal is negative, when no voltage is applied, liquid crystal molecules may be oriented in a direction along which the liquid crystal molecules are made orthogonal to said first substrate and second substrate.
In this case, it is preferable that said liquid crystal layer is given a pre-tilt angle in advance in the direction along which the liquid crystal molecules fall down when voltage is applied.
Also, in said liquid crystal layer, the anisotropy of the dielectric constant of liquid crystal is positive, and it may be such that it has a twisted nematic structure when having good symmetry can be naturally secured. Also, a chiral agent may be provided in the liquid crystal layer. In this case, two-divided TNs having only a different rise direction are secured, wherein it becomes possible to orient and divide liquid crystal in the pixels.
In addition, in said liquid crystal layer, the anisotropy of the dielectric constant of liquid crystal is positive, and the liquid crystal layer has a homogeneous structure when no voltage is applied. Said liquid crystal layer of the respective pixels has two or more types of minute areas in which the rise directions of the liquid crystal molecules differ from each other. In this case, it is preferable that the pre-tilt angle of the liquid crystal molecules on the boundary phase between said first substrate and second substrate is 1 degree or less.
In this case, the pixel electrode is shaped so as to have good symmetry, and the common electrode covers the entirety of the upper part of the pixel electrode when being observed from above, and is formed so that its area becomes larger than that of the pixel electrode. If voltage is applied between the pixel electrode and the common electrode, an oblique electric field is produced with good symmetry secured. Since the direction orientation of liquid crystal is regulated on the boundary phase between the substrates, two types of domains in which the rise directions differ from each other are produced. In the case of homogeneous orientation, it is preferable that a recess is provided at the middle of the pixel electrode particularly in order to stabilize the boundary phase.
A method for producing a reflection liquid crystal display according to the invention comprises the steps of: forming a thin film transistor on a first substrate; forming an optical absorption layer on said first substrate; forming a color filtering layer consisting of a cholesteric material layer on said optical absorption layer; forming a pixel electrode on said color filtering layer and connecting the same to said thin-film transistor; forming a common electrode on a second substrate; making a pixel electrode of said first substrate and a common electrode of said second substrate be opposed to each other, and forming a liquid crystal layer including a macromolecular organic compound between said first substrate and second substrate; forming a quarter-wavelength plate on said second substrate; and forming a polarization plate on said quarter-wavelength plate; wherein the process of forming said liquid crystal layer includes a process for pouring liquid crystal including monomer or oligomer between said first substrate and second substrate, and a process of making said monomer and oligomer macromolecular in liquid crystal.
In the invention, by making polymerizable monomer or oligomer, which is mixed in liquid crystal at a slight ratio, the macromolecular organic compound after the initial orientation is controlled by applying voltage between the common electrode and pixel electrode, whereby the beginning liquid crystal orientation can be made further secure. When controlling the initial orientation, the temperature may be lowered while applying voltage between the common electrode and pixel electrode after the liquid crystal layer is made isotropic by heating, or it is sufficient that voltage may be applied between the common electrode and pixel electrode at room temperature. Also, a reaction of making the monomer macromolecular may be carried out before and during heating in an isotropic phase, and after cooling down. Where voltage is applied between the common electrode and pixel electrode at room temperature and initial orientation is controlled, the reaction may be caused to occur before or after voltage is applied. Thus, it is possible to orient and divide liquid crystal in the form of normal drive.
A process of forming a pre-tilt angle in liquid crystal molecules by light irradiation may be provided after the process of forming said liquid crystal layer.
In addition, said light irradiation may be carried out by oblique irradiation of light with respect to said first substrate and second substrate, by oblique irradiation of polarized light with respect to said first substrate and second substrate, or by irradiation of polarized light from the perpendicular direction with respect to said first substrate and second substrate. Thereby, the pre-tilt angle is controlled in advance on the substrates in compliance with a division shape by using a method of orienting light, etc., wherein the control of the initial orientation can be remarkably and securely carried out. And, an effect of an oblique electric field and an effect of the pre-tilt angle synergetically operate, wherein the division orientation can be remarkably more effectively carried out than in the case of only either of them.
A method for driving a reflection liquid crystal display according to the invention is featured in that the display can dot-invertedly drive in a reflection liquid crystal display described in any one of claims 1 through 25.
In the invention, there is no problem in normal cases if the interval between pixels is sufficiently separated from each other as regards division. However, especially, in a case where pixels approach each other due to limitations in design, a state of generation of an oblique electric field is made further preferable by carrying out so-called dot-inverted drive in which the polarities (positive and negative) of the voltage applied onto respective pixels adjacent to each other are inverted when driving the display, whereby more satisfactory division is enabled.
A method for driving another reflection liquid crystal display according to the invention is featured in that a black state is enabled before one frame is finished, in a reflection liquid crystal display described in any one of claims 1 through 25, wherein separation or cut of moving pictures can be further improved.